1. Field of the Invention
This invention relates to an optical measuring apparatus used, for example, in a calibration of an apparatus such as a liquid crystal display panel (hereinafter abbreviated as LCD panel).
2. Description of the Related Art
An optical measuring apparatus has conventionally been used in a manufacturing line of an LCD panel for measuring optical characteristics such as chromaticity and luminance. Adjustment of white balance, quality control and shipping inspection of the LCD panel are executed by using the result of the measurement of the optical characteristics.
A conventional optical measuring apparatus such as the LCD color analyzer CA-110 produced by MINOLTA CO., LTD. comprises a probe and a main unit. The probe includes a photosensor for receiving rays emitted from a region of an LCD panel which is to be measured (hereinafter abbreviated as measurement region). The main unit calculates xyY (chromaticity coordinates and luminance) and T.DELTA.uvY (correlated color temperature, color difference and luminance), which are established by Commission Internationale de I'Eclairage (CIE), by basing on output of the photosensor of the probe. The calculated values are displayed on a display panel of the main unit.
The above-mentioned conventional optical measuring apparatus for the LCD panel is a noncontact type apparatus, in which the probe is positioned distant by a predetermined distance from the LCD panel. It is necessary to position the probe accurately at the position distant from the predetermined distance from the LCD panel.
Furthermore, in the measurement using the noncontact type optical measuring apparatus, rays of ambient illumination are reflected by a surface of the LCD panel, so that the reflected rays can enter into the photosensor of the probe as stray rays. When the stray rays enter into the probe, that is, the reflected rays of the ambient illumination are included in rays directly emitted from the measurement region of the LCD panel, output value from the photosensor of the probe is varied. As a result, the optical characteristics of the LCD panel are not estimated accurately. Especially, when the luminance of the LCD panel is relatively low, the stray rays largely affects the accuracy of the measurement. For solving this problem, it is necessary to reduce the ambient illumination of the prove and the LCD panel.
In another conventional optical measuring apparatus such as the CRT color analyzer CA-100 produced by MINOLTA CO., LTD., which is used for measuring optical characteristics of a CRT (Cathode Ray Tube) apparatus, a probe is directly contacted on a measurement region on The CRT apparatus. When this contact type apparatus is used for measuring the optical characteristics of the LCD panel, it causes the following problems.
The LCD panel is configured by that a liquid crystal layer is disposed between two glass plates for making the thickness of the liquid crystal layer even. When the probe is directly contacted on the surface of the LCD panel, the glass plate to which the probe is contacted is warped, so that the thickness of the liquid crystal layer in a region around the contacting portion of the probe and the LCD panel is varied. When the thickness of the liquid crystal layer is varied, colors of an image displayed on the region of the LCD panel is varied. In this case, the region where the thickness of the liquid crystal layer is varied corresponds to the measurement region. Thus, it is substantially impossible to measure the optical characteristics of the LCD panel by the conventional contact type apparatus.
On the other hand, it is conventionally known that the chromaticity and the luminance of the image displayed on the LCD panel varies when an observer moves from the front center of the LCD panel to the side thereof. This phenomenon is caused by light distribution of the LCD panel, that is, the larger the exit angle of the rays emitted from the LCD panel becomes, the smaller the intensity of the light becomes.
A light distribution of a typical LCD panel on the market is shown in FIG. 19A. The light distribution of the LCD panel is substantially symmetrical with respect to the normal "N" at the center of the LCD panel 12 in both of the vertical direction XX and the horizontal direction YY, so that an image can be displayed preferably when it is observed from the front center thereof. However, when the LCD panel is observed from the side at an observing angle larger than a predetermined angle .alpha. against the normal "N", the chromaticity and the luminance of the image are largely varied. It is generally said that the angle of view of the LCD panel is narrow.
In the conventional noncontact type optical measuring apparatus, the probe is positioned distant from the LCD panel, so that the probe receives not only paraxial rays but also the rays having exit angles larger than the predetermined angle .alpha. (in the following description, the angle .alpha. is called "maximum exit angle"). Thus, the accuracy and the repeatability of the measurement of the optical characteristics of the LCD panel by the conventional optical measuring apparatus are not so high.
Furthermore, in a kind of the LCD panel, the symmetry of the light distribution against the normal "N" will be deteriorated as shown in FIG. 19B when the display of the image is changed. For example, when the color of the image is gradually changed from white to black, it is observed that an exit angle of rays against the normal "N", at which the luminance of the image becomes the largest, increases. The variation of the light distribution of the LCD panel on the market is generally restricted only in the vertical direction XX in the figure. In this description, the spatial oblique coordinates are designated by symbols XX, YY and ZZ for distinguishing from tristimulus values X, Y and Z.
As shown in FIG. 20, even when the photosensor R of the probe is disposed on the normal "N" at the center of the LCD panel 12 for receiving rays emitted from the measurement region 121, the output S.sub.R from the photosensor R varies corresponding to the variation of the light distribution of the LCD panel 12.